1. Field of the Invention
This invention relates to an improved blown tube collapse frame assembly comprising a trapezoid section to change the cross section of a circular tube of thermoplastic material passing therethrough to a square cross section, and a transition section to pass the material from the square section to a flattened section between a pair of nip rolls with minimal material stretching in the process.
2. DESCRIPTION OF THE PRIOR ART
The blown tube process of producing thermoplastic film is well known in the prior art. One example of such a process is found in U.S. Pat. No. 2,720,680, Gerow, wherein a thermoplastic material is melt extruded in tubular form and the tubular form is retained by means of internal air pressure. The tube is passed between a pair of nip rolls to be wound into strips or sheeting. As taught in the Gerow disclosure, the pressure within the tube may be increased to expand the tube or to biaxially orient the thermoplastic material of the tube or bubble.
One of the principal problems in the blown tube process relates to the apparatus utilized to collapse the bubble or tube from a circular cross section to a flat cross section and from the flat section to a theoretical line between a pair of nip rolls, to be thereafter slit in film strip or sheet form. A typical prior art mechanical arrangement to provide for the collapse of a blown tube is illustrated in U.S. Pat. No. 3,235,632, Lemmer. In the typical collapsing arrangement of Lemmer, the blown tube is caused to pass through a V shape or A frame system of rollers which collapse the blown tube on two sides, i.e., two dimensional, from a circular cross section to a flat cross section. The collapse of such a tube section immediately from a circular cross section to a flat cross section causes uneven stresses to be imposed on or set up in the film to cause stretching and bagging of the final film which is most undesirable in many applications particularly for wound capacitor roll dielectrics which require a significantly flat or lay flat film.